In the submillimeter range, from 345 to 410 GHz, 472 new rotational transitions in the vibrational ground state of hydrazine have been measured. The majority of recorded lines form eight K=2<-1 Q-branches with J=<30, each branch being split due to internal rotation in hydrazine. New rotational transitions together with 443 known earlier have been fitted to an effective group-theoretical Hamiltonian originally developed by Hougen (J. T. Hougen, J. Mol. Spectrosc.89, 296-327 (1981)). A reasonable fit was obtained using several ΔK=0, +/-1, +/-2, and +/-4 matrix elements. For all 915 rotational transitions an overall standard deviation of the fit of 0.59 MHz was obtained using 32 parameters. These were 14 asymmetric rotor constants, 16 NH 2 inversion parameters, and 2 internal rotation parameters.